1. The composition dependence of the light scattering intensity of mixtures of each of four proteins, BSA, ovalbumin, ovomucoid, and soybean trypsin inhibitor, at concentrations up to 2 mg/ml, and a small molecule, sucrose, at concentrations of up to 200 g/l was measured using a technique and instrumentation developed in this laboratory and described in previous reports. The results were analyzed using formalism developed in this laboratory appropriate for binary mixtures of non-associating solutes exhibiting repulsive self- and hetero-interaction at high concentration. The results could be quantitatively accounted for by a semiempirical model, according to which the protein and sucrose are represented as equivalent spherical particles, the effective volume of which depends upon self- and hetero-interactions. The results indicate that sucrose interacts with itself and with proteins through a combination of steric repulsion and weak attraction that partially compensates for the steric repulsion. The magnitude of the attractive interaction between sucrose and protein varies between proteins, and is greatest for soybean trypsin inhibitor and smallest for ovalbumin (D. Wu). 2. The self-association of the bacterial septation protein FtsZ in the presence of GTP or a GTP analogue, GMPCPP, and 500 mM potassium was studied as a function of protein and Mg concentration. Data obtained from several biophysical measurements, including static and dynamic light scattering, sedimentation velocity, and fluorescence correlation spectroscopy reveal that with increasing concentration, the protein undergoes a concerted formation, akin to a second-order phase transition to a narrow distribution of high molecular weight oligomers with a stoichiometry of ca 80 (GTP) or 120 (GMPCPP). Above the transition concentration, the amount, but not the size, of oligomer increases, while that of low molecular weight species remains roughly constant (Monterroso, Rivas). All of the data obtained from the several types of measurements may be accounted for semiquantitatively in the context of a model, according to which the stable oligomer is a cyclic species, the size of which reflects the tendency of the protein fibrils to flex or curve in solution in the presence of different concentrations of Mg and the particular guanine nucleotide present (B. Monterroso, G. Rivas, CIB).